The correct answer is option 4, Digboi. Digboi is the oldest oil field in India and is located in the state of Assam. It was discovered in 1889 by the Assam Oil Company, making it the first commercial oil well in India. Digboi has a rich history in the Indian oil industry and has played a significant role in the country`s oil production. It has been continuously producing oil for over a century and is known for its high-quality crude oil. Other options mentioned in the question, namely Haldia, Bombay High, and Neyveli, are not the oldest oil fields in India.